Method of producing electrically semiconductive material



United States Patent 3,549,561 METHOD OF PRODUCING ELECTRICALLY SEMICONDUCTIVE MATERIAL Willem Westerveld and Jan Willem Harmsen, Em-

masingel, Eindhoven, Netherlands, assignors to US. Philips Corporation, New York, N.Y., a corporation of Delaware No Drawing. Continuation of application Ser. No.

705,289, Feb. 14, 1968, which is a continuation-inpart of application Ser. No. 395,037, Sept. 8, 1964. This application July 22, 1969, Ser. No. 849,234 Claims priority, application Netherlands, Sept. 27, 1963,

Int. Cl. H01b 1/06 US. Cl. 252-520 8 Claims ABSTRACT OF THE DISCLOSURE Produce electrically semiconductive material of improved temperature stability by adding from 1 to 4% by weight of silicon dioxide to a mixture consisting mostly of magnesium oxide and about 1.5% to 2.5% of titanium dioxide and sintering the mixture in a reducing atmosphere.

This application is a continuation of application Ser. No. 705,289 filed Feb. 14, 1968, now abandoned; and a continuation-impart of our copending patent application Ser. No. 395,037, filed Sept. 8, 1964, now abandoned.

The invention relates to a method of producing an electrically semiconductive material and to a semiconductor made from this material.

As is known, electrically semiconductive material having a small temperature coeflicient of resistance may be made from a mixture of an insulating oxide, such as magnesium oxide, and at most 3% of an oxide of one of the metals titanium, vanadium or niobium, by sintering the mixture at a temperature above 1700 C. in a reducing atmosphere.

It is also known to produce electrically semiconductive material by sintering a mixture consisting of at most 75% of titanium oxide, the remainder being an oxide of an alkaline earth metal or an oxide of aluminum, silicon, hafnium, thorium, tantalum or manganese, or a mixture of at least two of these oxides, in a reducing atmosphere.

However, it has been found that electrical resistors made of semiconductor material .do not perform satisfactorily at high temperatures. This applies particularly to their use as auxiliary resistors for limiting the ignition voltage in high-pressure mercury-vapor lamps, in Which the resistors are employed in the outer envelopes of the lamps and are exposed to high temperatures (up to 60 C.) for long periods of time (up to approximately 10,000 hours). When used for this purpose the resistance value of a semiconductor resistor must remain substantially constant and vary by no more than about 10% during the said period under the given conditions.

It has now been found that semiconductors satisfying the aforementioned stringent requirements can be produced from semiconductor material obtained by sintering under reducing conditions a mixture consisting mainly of magnesiumoxide and containing from 0.5% to 3% by weight of titanium dioxide and from 1% to 30% by weight, preferably from 1.3% to 2.0% by weight, of silicon dioxide.

The invention relates to a method of producing electrically semiconductive material by sintering in a reducing atmosphere a mixture consisting mainly of magnesium oxide and containing titanum doxide, and is characterized in that the mixture contains from 0.5% to 3.0% by weight of titanium dioxide and from 1.0% to 3.0% by weight of silicon dioxide, the proportion of both oxides together ice amounting to from 2% to 5% by weight. The specific resistivity of these materials ranges from about 300 to 3000 ohm/cm.

It has been found that by the method in accordance with the invention, semiconductors of high temperature stability, i.e. semiconductors the resistance values of which change hardly or not at all when they are exposed to high temperatures for long periods of time, are obtained by starting from a mixture containing from 2.5% to 4.0% by weight and particularly from 2.8% to 3.6% by weight of titanium dioxide and silicon dioxide taken together, and especially if the proportion of titanium dioxide is from 1.5% to 2.5% by weight and the proportion of silicon dioxide is from 1.3 to 2.0% by weight and more particularly if these proportions are approximately 1.7% by weight and 1.4% by weight respectively.

As a starting material magnesium oxide may be capable of producing magnesium oxide upon heating, for example magnesium carbonate. In the starting mixture, titanium dioxide and silicon dioxide may be present as such or entirely or in part, in combined form, for example, as magnesium titanate and magnesium silicate respectively. However, the use of at least one oxide and preferably of both oxides as such is preferred. In these cases the sintering process yields semiconductors having very satisfactory chemical properties.

In carrying out the method in accordance with the invention the starting mixture preferably contains, in addition to the aforementioned components, an organic binder, such as methyl cellulose or nitrocellulose. In such a case, before the mixture is sintered in a reducing atmosphere, the binder may be removed by heating to a lower temperature, for example, to a temperature of from 1300 C. to 1400 C. During this lower temperature heating process a small amount of sintering takes place.

It has been found that semiconductors of reproducible resistance value are especially obtained if this heating step is performed in an oxidizing atmosphere, for example, in

air.

The process of sintering in a reducing atmosphere may be effected in a gas mixture consisting of approximately by volume of nitrogen and approximately 15% by volume of hydrogen containing up to 0.1% by volume and preferably less than 0.01 by volume of oxygen, at temperatures between 1700 C., and 2000" C. and preferably by heatin g at about 1800 C., for example, for about from 5 to 30 minutes.

The method in accordance with the invention will now the described more fully with reference to the following example.

An intimate mixture of 96.5 parts by weight of magnesium oxide, 2.0 parts by weight of titanium dioxide, 1.5 parts by weight of silicon dioxide and 25 parts by weight of a solution of 10 grams of nitrocellulose and grams of ethylene glycol was compressed to form rods. The rods were dried in air and subsequently heated in air to approximately 1300 C. for minutes. The rods were then heated to a temperature of about 1800 C. in an atomsphere consisting of approximately 85% by volume of nitrogen, approximately 15% by volume of hydro gen and 0.003% by volume of oxygen for 20 minutes. To enable the resulting semiconductor rods to be used as auxiliary resistors in high-pressure mercury-vapor lamps electrodes were provided in a known manner. The resultant rods had a specific resistivity of 600 ohms-cm.

The temperature coefiicient of resistance of the rods is rather small; by increasing the temperature from 20 C. to 350 C. the resistance increases by about 50% in total, between 350 C. and 500 C. the resistance remains almost constant and above 500 C. the temperature coefficient of resistance shows a small negative value.

Resistances containing less than 1% by 'weight of silicon dioxide show at temperatures above about 500 C. a negative temperature coefiicient of relatively high value. Due thereto such resistances cannot be used in the highpressure mercury-vapor lamps as referred to above.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In the method of producing an electrically semiconductive material by sintering in a reducing atmosphere, at a tempertaure of about 1700 C. to about 2000 C., a mixture consisting essentially of magnesium oxide in the major proportion and 0.5% to 3.0% by Weight of titanium dioxide the improvement which comprises adding to said mixture silicon dioxide in such an amount that the mixture contains 1.0% to 3.0% by weight of silicon dioxide, and from 2% to 5% by weight of silicon dioxide and titanium dioxide in total, thereby producing an electrically semiconductive material of an improved electrical resistance-temperature relationship and an increased high temperature stability.

2. The method of claim 1, wherein in that the sum of the proportions of titanium dioxide and silicon dioxide 4 in the mixture amounts to from 3.5% to 4.0% by weight.

3. The method of claim 2, wherein the sum of the proportions of titanium dioxide and silicon dioxide in the mixture amounts to from 2.8% to 3.6% by weight.

4. The method of claim 1 wherein the mixture contains from 1.5% to 2.5% by weight of titanium dioxide and from 1.3% to 2.0% by weight of silicon dioxide.

5. The method of claim 4, wherein the mixture contains approximately 1.7% by Weight of titanium dioxide and approximately 1.4 by weight of silicon dioxide.

6. The method of claim 1 wherein an organic binder is present in the mixture before sintering.

7. The method of claim 6 wherein before the mixture is sintered in a reducing atmosphere it is heated in an oxidizing atmosphere to a temperature of from 1300 C. to 1400 C.

8. An electric semiconductor made of a material produoed by the method of claim 1.

References Cited UNITED STATES PATENTS 2,289,211 7/1942 Ridgway 252520 2,507,233 5/1950 Verwey 252-520 3,036,018 5/1962 Peras 252520 DOUGLAS J. DRUMMOND, Primary Examiner 

